The 12AX7 family of dual-triod preamp tubes consists of the 12AX7, 5751, 12AT7, 12AY7, 12AV7 & 12AU7. These are all pin compatable with one another, the only differences being the gain factor of each tube. A common substitution is to replace a 12AX7 with a 5751 or a 12AT7 to tame a preamp that tends to overdrive too easily, allowing you to get a better 'clean' sound out of your amp.
It is important to note that the above is not carved in stone and basically any of these tubes can be substituted for any other. You can always replacing a higher-gain tube with a lower-gain tube, using your ears to tell you whether this was good idea or not. As example of this, my Fender Pro Junior amp came stock with two 12AX7 tubes - one for the preamp and one for the inverter. I have since re-tubed it with a 12AU7 in the preamp stage and a 12AT7 for the inverter. The 12AU7 in the preamp gives a nice clean sound, and the 12AT7 inverter helps prevent overdriving the power amp.